65 lines
2.4 KiB
Markdown
65 lines
2.4 KiB
Markdown
# Thought dump
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### Data + Vtable
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We need to get a basic grasp of this to be able to understand how we implement a Waker.
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Next concept is `Pin`. What guarantee do we give/get?
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* Not too deep into `Pin`, the real need for this trait relates to the internals of `Future`and Async/Await implementation which is another 200 lines of .... book. Interesting but really an implementation detail.
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* Next book, talk about dtolnay's `await!`macro. Check if it's feasable to make our own to see what the compiler really does. Why `Pin`is needed.
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### TODO:
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* Find Rfc's to point to for more information about concepts. also to double check my conclusions.
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Run this code in the playground:
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```rust
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use std::rc::Rc;
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fn main() {
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use std::mem::size_of;
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println!("Size of Box<i32>: {}", size_of::<Box<i32>>());
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println!("Size of &i32: {}", size_of::<&i32>());
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println!("Size of &Box<i32>: {}", size_of::<&Box<i32>>());
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println!("Size of Box<Trait>: {}", size_of::<Box<MyTrait>>());
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println!("Size of &dyn Trait: {}", size_of::<&dyn MyTrait>());
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println!("Size of Rc<i32>: {}", size_of::<Rc<i32>>());
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println!("Size of Box<Rc<i32>>: {}", size_of::<Box<Rc<i32>>>());
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println!("Size of Rc<Trait>: {}", size_of::<Rc<MyTrait>>());
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println!("Size of &[i32]: {}", size_of::<&[i32]>());
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println!("Size of &[&dyn Trait]: {}", size_of::<&[&dyn MyTrait]>());
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println!("Size of [i32; 10]: {}", size_of::<[i32; 10]>());
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println!("Size of [&dyn Trait; 10]: {}", size_of::<[&dyn MyTrait; 10]>());
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}
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trait MyTrait {
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fn do_something(&self) {
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println!("See, something");
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}
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}
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```
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### Smart pointers = Data + Vtable
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### Dynamic Dispatch - short intro
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For more information about this:
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{% embed url="https://alschwalm.com/blog/static/2017/03/07/exploring-dynamic-dispatch-in-rust/" %}
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Create our own "fat pointer". We need to do this when we create Wakers anyway so let's get to know them a little. It's probably the thing that's most difficult implementation wise.
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### Bonus - pointer types
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Normal pointer: just a memory location. Basic pointer type we all know \(reference/pointer\)
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Fat pointer: pointer plus some other information \(slice length, a second pointer to trait information for a trait object\). 16 byte pointer, not only compiler "hint".
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Smart pointer: a pointer-like type that add behaviour or restrictions eg `Box` deallocates when dropped, `Rc` tracks how many shared owner there currently are. Compiler only!
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